Pulse Oximetry (PO) is a non-invasive method for estimating arterial oxygen saturation from Photoplethysmography signals recorded from peripheral tissue sites. Despite a very wide range of clinical applications, the fundamental working principles of PO is still not very well understood. In this work, a Monte Carlo model is executed for a monolayer volume of human skin dermis in a reflectance detection modality to investigate the basic nature of light-tissue interaction in the tissue. Differences in systolic and diastolic blood volume explained the pulsatility. The distribution of light-tissue interaction was illustrated, systolic and diastolic reflectance at red and infrared light were simulated and the 'calibration curve' was produced
